Wheat is an important and strategic cereal crop for the majority of the world's populations. It is the most important staple food of about two billion people (36% of the world population). Worldwide, wheat provides nearly 55% of the carbohydrates and 20% of the food calories consumed globally. It exceeds in acreage and production every other grain crop (including rice, maize, etc.) and is therefore, the most important cereal grain crop of the world, which is cultivated over a wide range of climatic conditions. The understanding of genetics and genome organization using molecular markers is of great value for genetic and plant breeding purposes.
The world's main wheat producing regions are China, India, United States, Russian Federation, France, Australia, Germany, Ukraine, Canada, Turkey, Pakistan, Argentina, Kazakhstan and United Kingdom. Most of the currently cultivated wheat varieties belong Triticum aestivum L., which is known as common bread wheat and valued for bread making. The greatest portion of the wheat flour produced is used for bread making.
Bread wheat is a hexaploid, with three complete genomes termed A, B and D in the nucleus of each cell. Each of these genomes is almost twice the size of the human genome and consists of around 5,500 million nucleotides. Durum wheat, also known as macaroni wheat or pasta wheat (Triticum durum or Triticum turgidum subsp. durum), is the major tetraploid species of wheat of commercial importance, which is widely cultivated today. Durum wheat has two complete genomes, A and B, and is widely used for making pasta.
Wheat is a widely studied plant, but in some cases, development of new traits is hampered by limited genetic diversity in today's commercial wheat cultivars and also because the bread wheat genome typically has three functionally redundant copies of each gene (called homoeologs), and therefore, single gene alterations usually do not produce any readily visible phenotype such as those that have been found in diploid corn. Often in bread wheat, altered variants of all three homoeologs must be combined genetically in order to evaluate their effects.
Improving the shelf life of whole grain flour is important to meet the increasing food demands of the world population. Whole grain products offer many health advantages such as reducing the risk of chronic diseases such as coronary heart disease, type 2 diabetes, and some types of cancer. Whole grain products can also help improve body weight management and digestive health. Despite these health benefits, greater than 95% of the United States population consume below the recommended daily allowance of whole grains. Consumption of whole grain products are lower due to the bitter and off flavors that develop more rapidly in whole grain flour due to the susceptibility of its lipid fraction to hydrolytic and oxidative rancidity by lipases, lipoxygenases and other enzymes. Improving shelf-life and sensory characteristics of whole grain flour and food products by improving oxidative stability could positively affect consumer acceptance of whole grain products.
Lipoxygenase (Lpx), linoleate: oxygen oxidoreductase; (EC 1.13.11.12) is a class of non-heme iron-containing dioxygenases that catalyse the positional and specific dioxygenation of polyunsaturated fatty acids that contain 1,4-cis,cis pentadiene structures to produce the corresponding hydroperoxides. Lpx are key enzymes catalyzing the oxidation of polyunsaturated fatty acids. Lpxs are non-heme iron-containing dioxygenases, and are monomeric proteins with molecular mass ranging from 94 to 105 kDa in plants. There are many Lpx genes in plant genomes. For example, the Arabidopsis genome contains 6 Lpx genes and the rice genome contains 14 Lpx genes. Wheat, which has a genome size 108 times larger than Arabidopsis and 36 times larger than rice, has not yet been fully characterized for Lpx genes. At least 3 known wheat Lpx gene families, each with at least one homoeolog on the A, B and D genomes, have been identified to date. Lpx1 and Lpx3 are on chromosome 4 and Lpx2 is on chromosome 5. A quantitative trait locus for lipoxygenase activity has also recently been reported on chromosome 1A in durum wheat.
In plants, products of the lipoxygenase reaction have been shown to have roles in several processes, such as vegetative growth, wounding, response to herbivore and pathogen attack and also mobilization of storage lipids during germination. In rice, double mutants of two different genes, Lox1 and Lox2, but not single mutations in Lox3, improved germination and stability of intact grains for up to 42 months.
In durum wheat, radicals produced during the intermediate states of polyunsaturated fatty acid hydroperoxidation can cause oxidation of carotenoid pigments, and consequently a loss of the yellow flour color preferred for pasta products. Wheat lipoxygenases have been characterized in durum wheat due to efforts to increase yellow carotenoid levels in those varieties. A deletion allele in the durum wheat LpxB1.1 gene (called Lpx-B1.1c) in particular was found associated with improved yellow color in pasta products. Durum wheat lines with low lipoxygenase activity were also associated with a reduction of Lpx-3 transcript levels in the late stages of grain filling.
In addition to the LpxB1.1 gene, the wheat B genome has an additional copy of the Lpx1 gene that is present either as LpxB1.2 or LpxB1.3. In both durum and bread wheat, the A genome Lpx1 gene is encoded by a pseudogene called LpxA1-like (GenBank FJ518909). Durum wheat does not have the D genome, but an Lpx1 gene in the D genome of bread wheat has also been identified, and the sequence recently deposited in GenBank (KC679302).
In bread wheat, lipases and lipoxygenases play a role in lipid degradation, which can contribute to wheat products with decreased nutritional quality, decreased functional properties and decreased sensory acceptability. Lipoxygenase activity in bread wheat leads to degradation of carotenoids and decreased nutritional value. Since multiple Lpx genes (Lpx1, 2 and 3) are all expressed in the wheat grain each with one or more potential homoeologs in the A, B and D genomes, it is unclear if altering one gene or gene family could positively affect oxidative stability of whole grain flour in bread wheat. Mutations in the lipoxygenase genes in the wheat genome provide a potential pathway for providing increased oxidative stability in wheat flour and products derived therefrom. The disclosure herein demonstrates that novel alleles in the Lpx1 gene significantly improve shelf-life of whole grain flour.